In order for a print imaging support to be widely accepted by the consumer for imaging applications, it has to meet requirements for preferred basis weight, caliper, stiffness, smoothness, gloss, whiteness, and opacity. Supports with properties outside the typical range for ‘imaging media’ suffer low consumer acceptance.
In addition to these fundamental requirements, imaging supports are also subject to other specific requirements depending upon the mode of image formation onto the support. For example, in the formation of photographic paper, it is important that the photographic paper be resistant to penetration by liquid processing chemicals, failing which, a stain is apparent on the print border accompanied by a severe loss in image quality. In the formation of ‘photo-quality’ ink jet paper, it is important that the paper is readily wetted by ink and that it exhibits the ability to absorb high concentrations of ink and dry quickly. If the ink is not absorbed quickly, the elements block (stick) together when stacked against subsequent prints and exhibit smudging and uneven print density. For thermal media, it is important that the support contain an insulative layer in order to maximize the transfer of dye from the donor, which results in a higher color saturation.
It is important, therefore, for an imaging media to simultaneously satisfy several requirements. One commonly used technique in the art for simultaneously satisfying multiple requirements is through the use of composite structures comprising multiple layers wherein each of the layers, either individually or synergistically, serves distinct functions. For example, it is known that a conventional photographic paper comprises a cellulose paper base or support that has applied thereto a layer of polyolefin resin, typically polyethylene, on each side, which serves to provide waterproofing to the paper and also provides a smooth surface on which the photosensitive layers are formed. In an imaging material described in U.S. Pat. No. 5,866,282, biaxially oriented polyolefin sheets are extrusion laminated to cellulose paper to create a support for silver halide imaging layers. The biaxially oriented sheets described therein have a microvoided layer in combination with coextruded layers that contain white pigments such as titanium dioxide above and below the microvoided layer. The composite imaging support structure described has been found to be more durable, sharper, and brighter than prior art photographic paper imaging supports that use cast melt extruded polyethylene layers coated on cellulose paper. In U.S. Pat. No. 5,851,651, porous coatings comprising inorganic pigments and anionic, organic binders are blade coated to cellulose paper to create ‘photo-quality’ ink jet paper.
In all of the above imaging supports, multiple operations are required to manufacture and assemble the individual layers into a support. For example, photographic paper typically requires a paper-making operation followed by a polyethylene extrusion coating operation, or as disclosed in U.S. Pat. No. 5,866,282, a paper-making operation is followed by a lamination operation for which the laminates are made in yet another extrusion casting operation. There is a need for imaging supports that can be manufactured in a single in-line manufacturing process while still meeting the stringent features and quality requirements of imaging supports.
It is also well known in the art that traditional imaging supports consist of raw paper base. For example, in typical photographic paper as currently made, approximately 75% of the weight of the photographic paper comprise the raw paper base. Although raw paper base is typically a high modulus, low cost material, there exist significant environmental issues with the paper manufacturing process. There is a need for alternate raw materials and manufacturing processes that are more environmentally friendly. Additionally to minimize environmental impact, it is important to reduce the raw paper base content, where possible, without sacrificing the imaging support features that are valued by the customer, that is, strength, stiffness, surface properties of the imaging support.
An important corollary of the above is the ability to recycle photographic paper. Current photographic papers cannot be recycled because they are composites of polyethylene and raw paper base and, as such, cannot be recycled using polymer recovery processes or paper recovery processes. A photographic paper that comprises significantly higher contents of polymer lends itself to recycling using polymer recovery processes.
Existing composite color paper structures are typically subject to curl through the manufacturing, finishing, and processing operations. This curl is primarily due to internal stresses that are built into the various layers of the composite structure during manufacturing and drying operations, as well as during storage operations (core-set curl). Additionally, since the different layers of the composite structure exhibit different susceptibility to humidity, the curl of the imaging support changes as a function of the humidity of its immediate environment. There is a need for an imaging support that minimizes curl sensitivity as a function of humidity, or ideally, does not exhibit curl sensitivity.
In papers having a backside polyolefin coating, an absence of titanium dioxide in the backside polyolefin coating results in the print indicia appearing clear and the overall required total opacity is achieved in general by the filling of the raw-base paper and a corresponding titanium dioxide addition into the polyethylene coating of the front face side. However, with some imaging base papers, there is required such a high opacity that a titanium dioxide addition is also required in the backside coating. Based on this titanium dioxide addition, an interference with the legibility of the print picture occurs. There is a need for a high opacity imaging base which avoids this legibility interference.
The stringent and varied requirements of imaging media, therefore, demand a constant evolution of material and processing technology. One such technology known in the art as ‘polymer foams’ has previously found significant application in food and drink containers, packaging, furniture, and appliances. Polymer foams have also been referred to as cellular polymers, foamed plastic, or expanded plastic. Polymer foams are multiple phase systems comprising a solid polymer matrix that is continuous and a gas phase. For example, U.S. Pat. No. 4,832,775 discloses a composite foam/film structure which comprises a polystyrene foam substrate, oriented polypropylene film applied to at least one major surface of the polystyrene foam substrate, and an acrylic adhesive component securing the polypropylene film to the major surface of the polystyrene foam substrate. The foregoing composite foam/film structure may be shaped by conventional processes as thermoforming to provide numerous types of useful articles including cups, bowls, and plates, as well as cartons and containers that exhibit excellent levels of puncture, flex-crack, grease and abrasion resistance, moisture barrier properties, and resiliency.
Foams have also found limited application in imaging media. For example, JP 2839905 B2 discloses a 3-layer structure comprising a foamed polyolefin layer on the image-receiving side, raw paper base, and a polyethylene resin coat on the backside. The foamed resin layer was created by extruding a mixture of 20 weight % titanium dioxide master batch in low density polyethylene, 78 weight % polypropylene, and 2 weight % of Daiblow PE-M20 (AL)NK blowing agent through a T-die. This foamed sheet was then laminated to the paper support using a hot melt adhesive. The disclosure JP 09127648 A highlights a variation of the JP 2839905 B2 structure, in which the resin on the backside of the paper support is foamed, while the image receiving side resin layer is unfoamed. Another variation is a 4-layer structure highlighted in JP 09106038 A. In this, the image receiving resin layer comprises 2 layers, an unfoamed resin layer which is in contact with the emulsion, and a foamed resin layer which is adhered to the paper support. There are several problems with this, however. Structures described in the foregoing patents need to use foamed layers as thin as 10 μm to 45 μm, since the foamed resin layers are being used to replace existing resin coated layers on the paper support. The thickness restriction is further needed to maintain the structural integrity of the photographic paper support since the raw paper base is providing the stiffness. It is known by those versed in the art of foaming that it is very difficult to make thin uniform foamed films with substantial reduction in density especially in the thickness range noted above.
Several disadvantages occur when employing the conventional printing processes for this kind of characterizing and/or marking of imaging supports and in particular closed cell foam core imaging members. First, the closed cell imaging members have a non-porous surface and there are problems in printing them with conventional printing inks. With conventional paper based imaging members, the base paper is somewhat porous and the inks will quickly wet the surface and the water will be partly absorbed into the fiber structure of the paper. When printing a closed cell foam core member, the surface tension of the foam core is such that some volatile organic materials may have to be added to the ink formula. These materials may include high boiling alcohols that have a high vapor pressure thus requiring high temperatures be used to drive off these materials. This creates additional problem with environmental emissions as well as presenting additional difficulties to assure safe working conditions for the operators. Furthermore, the print indicia, such as characters, may generate interfering effects relating to the final photographic picture, generated on the emulsion-coated side, based on photo-chemical reactions.
Embossing the polyolefin-coated photographic base paper has already been tried for the application of indicia. For this purpose, the polyethylene-coated paper was passed through a calender, where the roller of the calender was furnished with a specially structured surface. This method has proven to be unsuitable for the application of indicia, since no uniform stamping and impression depth could be achieved. In addition, the method required two separate operations, coating and stamping, which could not be performed in one single operational step.
In addition, a method exists in the art, where a chill cylinder roller, with a specially prepared surface is employed for the application of different indicia, such as patterns or symbols, onto the back side of a polyolefin-coated base paper, which chill cylinder roller allows the polyolefin-extrusion coating and the characterization in one single operational step, in-line.
U.S. Pat. No. 5,160,777 proposes to provide a means of embossing the backside of a resin coated paper based imaging member. This patent discloses that, in order to emboss the backside of a resin coated paper base, limitation of the depth of the backside embossing and gloss differences between the print area and background are necessary to prevent transfer of the embossed indicia to the face side. Such limitations limit the usefulness of the embossing method because the ability to see embossed logo indicia largely depends on the spectral reflection difference between two areas with different roughnesses. There remains a need to provide a means and an imaging member which provides the necessary properties of conventional imaging supports that is less prone to embossed indicia transfer in order to provide better brand recognition and manufacturing quality control.